NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, selective catalytic reduction (SCR), and lean NOx traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean-burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
To clarify the state of a sometime ambiguous nomenclature, it should be noted that in the exhaust aftertreatment art, the terms “SCR catalyst” and “lean NOx catalyst” are occasionally used interchangeably. Where the term “SCR” is used to refer just to ammonia-SCR, as it often is, SCR is a special case of lean NOx catalysis. Commonly when both types of catalysts are discussed in one reference, SCR is used with reference to ammonia-SCR and lean NOx catalysis is used with reference to SCR with reductants other than ammonia, such as SCR with hydrocarbons.
LNTs are devices that adsorb NOx under lean exhaust conditions and reduce and release the adsorbed NOx under rich exhaust conditions. A LNT generally includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals, such as Pt and Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. In a reducing environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to regenerate (denitrate) the LNT.
Creating a reducing environment for LNT regeneration involves providing a reductant to the exhaust. Except where the engine can be run stoichiometric or rich, a portion of the reductant reacts within the exhaust to consume oxygen. The amount of oxygen to be removed by reaction with reductant can be reduced in various ways. If the engine is equipped with an intake air throttle, the throttle can be used. However, at least in the case of a diesel engine, it is generally necessary to eliminate some of the oxygen in the exhaust by combustion or reforming reactions with reductant that is injected into the exhaust.
The reactions between reductant and oxygen can take place in the LNT, but it is generally preferred for the reactions to occur in a catalyst upstream of the LNT, whereby the heat of reaction does not cause large temperature increases within the LNT at every regeneration.
Reductant can be injected into the exhaust by the engine or a separate fuel injection device. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. Alternatively, or in addition, reductant can be injected into the exhaust downstream of the engine.
U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the '037 publication”) describes an exhaust treatment system with a fuel reformer placed in the exhaust line upstream of a LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate.
The operation of an inline reformer can be modeled in terms of the following three equations:0.684CH1.85+O2→0.684CO2+0.632H2O  (1)0.316CH1.85+0.316H2O→0.316CO+0.608H2  (2)0.316CO+0.316H2O→0.316CO2+0.316H2  (3)wherein CH1.85 represents an exemplary reductant, such as diesel fuel, with a 1.85 ratio between carbon and hydrogen. Equation (1) is exothermic complete combustion by which oxygen is consumed. Equation (2) is endothermic steam reforming. Equation (3) is the water gas shift reaction, which is comparatively thermal neutral and is not of great importance in the present disclosure, as both CO and H2 are effective for regeneration.
The inline reformer of the '037 publication is designed to be rapidly heated and to then catalyze steam reforming. Temperatures from about 500 to about 700° C. are said to be required for effective reformate production by this reformer. These temperatures are substantially higher than typical diesel exhaust temperatures. The reformer is heated by injecting fuel at a rate that leaves the exhaust lean, whereby Reaction (1) takes place. After warm up, the fuel injection rate is increased to provide a rich exhaust. Depending on such factors as the exhaust oxygen concentration, the fuel injection rate, and the exhaust temperature, the reformer tends to either heat or cool as reformate is produced.
The fuel injection rate can be used to control the reformer temperature. If the reformer is heating, the fuel injection rate can be increased to increase the extent of endothermic Reaction (2) (endothermic steam reforming) occurs. The extent of Reaction (1) (exothermic complete combustion), which is limited by the exhaust oxygen concentration, remains essentially constant. If the reformer is cooling, the fuel injection rate can be decreased.
If the fuel injection rate alone is used to control the reformer temperature, the reductant concentration provided by the reformer will be essentially uncontrolled. In order to have some control over the reductant concentration, the exhaust oxygen concentration can be used as an additional control variable. The exhaust oxygen concentration in a diesel exhaust system can be controlled, within limits, using EGR, and intake air throttling. By controlling both the exhaust oxygen concentration and the fuel injection rate, the reformer temperature and the reductant concentration can be simultaneously controlled to predetermined values.
During denitrations, much of the adsorbed NOx is reduced to N2, however, a portion of the adsorbed NOx is released without having been reduced and another portion of the adsorbed NOx is deeply reduced to ammonia. The NOx release occurs primarily at the beginning of the regeneration and is described as a NOx release spike. The ammonia production has generally been observed mostly towards the end of the regeneration.
U.S. Pat. No. 6,732,507 proposes a system in which a SCR catalyst is configured downstream of a LNT in order to utilize the ammonia released during denitration. The ammonia is utilized to reduce NOx slipping past the LNT and thereby improves conversion efficiency over a stand-alone LNT with no increase in fuel penalty or precious metal usage. U.S. Pat. No. 6,732,507 proposes regenerating the LNT using more reductant than required to reduce the adsorbed NOx with the idea of fueling ammonia production and thereby realizing NOx reduction over the SCR catalyst.
U.S. Pat. Pub. No. 2004/0076565 (hereinafter “the '565 publication”) also describes hybrid systems combining LNT and SCR catalysts. In order to increase ammonia production, it is proposed to reduce the rhodium loading of the LNT. In order to reduce the NOx spike, it is proposed to eliminate oxygen storage capacity from the LNT. The theory is that the NOx spike results from reductant reacting with stored oxygen to produce heat, which causes the release of unreduced NOx at the beginning of each regeneration
WO 2005/049984 also describes systems having LNT and SCR catalysts. Theorizing that ammonia formation is prohibited by residual oxygen present within the LNT patent, this application describes a process in which the LNT is flushed with an inert exhaust, which is an essentially stoichiometric or slightly lean mixture, in order to reduce residual oxygen and increase the degree of ammonia production during LNT regeneration.
U.S. Pat. No. 5,778,677 theorizes that the NOx spike is the result of an imbalance between reductant supply rate and the NOx release rate at the beginning of each regeneration. The proposed solution is to treat the NOx released at the start of the regeneration in a downstream SCR catalyst. Ammonia is provided downstream of the LNT for this purpose. It is proposed that this ammonia be produced on board by processing rich exhaust through a three-way catalyst in a separate exhaust passage. The ammonia-containing exhaust is combined with the LNT effluent prior to the exhaust entering the SCR catalyst.
In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and is practical for reducing NOx emissions from diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.